The present invention relates to a current control device particularly for power circuits in MOS technology.
Power devices in MOS technology (MOS being the acronym of Metal Oxide Semiconductor) are currently used to drive loads with fixed or variable impedance; said devices have various configurations, the most interesting of which, from a strictly applicative point of view, are those known in the art as high-side driver and low-side driver, i.e. driving circuits in which the load or impedance element respectively has an electrode or terminal connected to the negative pole of the power supply and has a terminal connected to the positive pole of the power supply.
Power devices for driving loads with fixed or variable impedance often require a circuit for controlling the delivered current. This control circuit assumes various functions, such as protecting the device from overloads, increasing the duration of the life of the load, and limiting electromagnetic emissions when switching the load on and/or off.
Power circuits usually comprise a final power stage constituted by an N-channel or P-channel MOSFET transistor. In the case of a high-side driver with an N-channel MOSFET transistor, the source terminal constitutes the output of the power circuit to which a terminal of the load is connected.
Providing current control in a monolithic integrated circuit without the aid of external components entails a problem which is difficult to solve, i.e. the problem of frequency stability. There are two different possibilities for providing frequency compensation in the current control loop which acts on the gate-source voltage of the MOSFET transistor. The first one is to drive said voltage with a low impedance, using a classic operational amplifier compensated by an internal capacitor. The other one entails high-impedance driving, using the gate-source capacitor which is intrinsic in the MOSFET transistor itself for compensation.
The first solution has a substantial problem, i.e. the provision of an integrated capacitor of a sufficiently high value, such as to produce a dominant pole. In this case, the use of the gate-source capacitor of the MOSFET transistor as load makes this aim very difficult, since the value of the gate-source capacitor is generally very high with respect to the integrated capacitors, which can attain at the most a few tens of picofarads, whereas the gate-source capacitor is proportional to the area occupied by the MOSFET transistor, which cannot be economically sacrificed merely to provide a compensation capacitor.
Compensation provided with the first solution is applied only in the case of very small MOSFET transistors, i.e. in the case of driving circuits for currents of a few milliamperes.
It has furthermore been observed so far that the second solution is also not excessively effective. Only circuits provided with physically very large MOSFET transistors, with gate-source capacitor values of several nanofarads, have in fact been found to be stable in practical application, whereas the necessary frequency stability is not achieved for integrated circuits having MOSFET transistors of intermediate dimensions. Therefore, the real technical problem consists of the intermediate capacitor values, which so far are not covered by the first described solution or by the second one.